The objectives of this project are to determine if titers of circulating antibody to mouse mammary tumor virus (MMTV) in women with breast cancer are useful in a) evaluating the effectiveness of therapy and b) detecting the recurrance of neoplastic breast disease. Sera will be analyzed by indirect immunofluorescnece (IF) and enzyme linked immunoassay (ELISA) for antibodies to mouse mammary tumor virus-related antigens. The antibody titers will be correlated with survival, onset of metastatic disease, response to various therapeutic regimens, and other personal, clinical and pathological factors. Studies will also be performed to determine if the limited number of patients with benign breast disease whose sera contain antibody to mouse mammary tumor antigen are at a higher risk of developing malignant breast disease than those in whom circulating antibody cannot be detected.